The clinical and laboratory findings are presented of three patients (two affected sisters and their male cousin) with myophosphorylase deficiency in two interrelated families. Nine unaffected relatives were also investigated. Although the three patients demonstrated the characteristic features of the disease, their unaffected relatives showed no clear evidence of a heterozygous state. The genetic findings support the hypothesis that the disease is inherited as a rare autosomal recessive. A possible sex-limited mode of inheritance is discussed.